feaLib: ensure languagesystems using 'DFLT' script tag precede all others

https://github.com/fonttools/fonttools/pull/1292#issuecomment-407427544

this is to match makeotf's behavior

https://github.com/adobe-type-tools/afdko/pull/498/files#diff-17824930b3cdfb707ae56e1f42bccec6R2305

Lib/fontTools/feaLib/builder.py

@@ -59,6 +59,7 @@ class Builder(object):
         self.lookupflag_ = 0
         self.lookupflag_markFilterSet_ = None
         self.language_systems = set()
+        self.seen_non_DFLT_script_ = False
         self.named_lookups_ = {}
         self.cur_lookup_ = None
         self.cur_lookup_name_ = None
@@ -616,6 +617,15 @@ class Builder(object):
             raise FeatureLibError(
                 'If "languagesystem DFLT dflt" is present, it must be '
                 'the first of the languagesystem statements', location)
+        if script == "DFLT":
+            if self.seen_non_DFLT_script_:
+                raise FeatureLibError(
+                    'languagesystems using the "DFLT" script tag must '
+                    "precede all other languagesystems",
+                    location
+                )
+        else:
+            self.seen_non_DFLT_script_ = True
         if (script, language) in self.default_language_systems_:
             raise FeatureLibError(
                 '"languagesystem %s %s" has already been specified' %

Tests/feaLib/builder_test.py

@@ -285,6 +285,17 @@ class BuilderTest(unittest.TestCase):
             "it must be the first of the languagesystem statements",
             self.build, "languagesystem latn TRK; languagesystem DFLT dflt;")
 
+    def test_languagesystem_DFLT_not_preceding(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            "languagesystems using the \"DFLT\" script tag must "
+            "precede all other languagesystems",
+            self.build,
+            "languagesystem DFLT dflt; "
+            "languagesystem latn dflt; "
+            "languagesystem DFLT fooo; "
+        )
+
     def test_script(self):
         builder = Builder(makeTTFont(), (None, None))
         builder.start_feature(location=None, name='test')